Indolicidin is comprised of 13 amino acids, 5 of which are tryptophan residues, and the carboxylterminal arginine is carboxamidated. Indolicidin has the highest tryptophan content of any known protein.

Indolicidin, a powerful antimicrobial peptide extracted from bovine neutrophil cytoplasmic granules, comprises 13 amino acids, including a notably high tryptophan content, with five tryptophan residues, and features a carboxamidated carboxylterminal arginine. This peptide, recognized for having the highest tryptophan level among proteins, also acts as a tridecapeptide amide exhibiting bactericidal activities rivaling those of prominent defensins and bactenecins in vitro. Its mechanism includes binding to surface lipopolysaccharide with high affinity, facilitating the permeabilization of Escherichia coli's outer membrane via the self-promoted uptake pathway, as evidenced by its interaction with 1-N-phenylnapthylamine. Furthermore, methyl esterification at Indolicidin's carboxyl terminus amplifies its effectiveness against both Gram-negative and Gram-positive bacteria, attributed to enhanced lipopolysaccharide binding and outer membrane permeabilization in Gram-negative bacteria, and pinpointing the cytoplasmic membrane as its action site through membrane permeabilization indicators in Escherichia coli.